Cmos sensor with shallow and deep regions

ABSTRACT

A CMOS sensor. The CMOS sensor comprises a substrate, a gate electrode formed on the substrate, a source/drain region formed in the substrate on one side of the gate electrode, and a sensor region formed in the substrate on another side of the gate electrode. The impurity in the source/drain region is arsenic. The source/drain further comprises a lightly doped drain region. The sensor region comprises a first doped region and a second doped region which together have a dentoid profile. The impurity in the first doped region and the second doped region is phosphorus.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates in general to a structure of semiconductorintegrated circuits (ICs), and more particularly to a structure of acomplementary metal-oxide semiconductor (CMOS) sensor.

[0003] 2. Description of the Related Art

[0004] Charge-coupled devices (CCDs) have been the mainstay ofconventional imaging circuits for converting light into an electricalsignal that represents the intensity of the energy. CCD applicationsinclude monitors, transcription machines and cameras. Although CCDs havemany strengths, CCDs use is restricted by their high cost and theirvolume. To reduce their cost, dimensions and energy consumption, a CMOSphoto diode device has been develop. Because a CMOS photo diode devicecan be produced using conventional techniques, the cost and the volumeof the sensor can be reduced. CMOS photo diode applications include PCcameras, digital cameras, etc.

[0005] A photo diode based on the theorem of a P-N junction can convertlight into an electrical signal. Before energy in the form of photonsstrikes the photo diode, there is an electric field in the P-N junction.The electrons in N region do not diffuse towards P region and the holesin P region do not diffuse towards N region. When enough light strikesthe photo diode, the light creates a number of electron-hole pairs. Theelectrons and the holes diffuse towards the P-N junction. When theelectrons and the holes reach the P-N junction as a result of the effectof the inner electric field across the junction, the electrons flow tothe N region and the holes flow to the P region. Thus a current isinduced between the P-N junction electrodes. Ideally, a photo diode inthe dark is an open-circuit. In other words there is no current inducedby light while a photo diode is in the dark.

[0006]FIG. 1 is a schematic, cross-sectional view of a portion of asemiconductor device showing a conventional CMOS sensor. In FIG. 1, theconventional CMOS sensor includes a P-type substrate 100, a field oxidelayer 104, a P-type well 110, a gate structure 120, an N-typesource/drain region 122, an N-type sensor region 124, an depletionregion 126, and a borophosphosilicate glass/silicon nitride glassdielectric layer 134.

[0007] When a light beam 140 passes through the depletion region 126which works as a P-N junction, the depletion region 126 is excited and anumber of electron-hole pairs are created. Thus the light is convertedinto an electric signal.

[0008] However, with respect to a CMOS image sensor, transmittance oflight for the semiconductor structure used in a semiconductor imagesensor is an important factor that seriously influences the quality ofthe image sensor. For example, it the imperative that the lighttransmittance is high enough. Only a high transmittance enables thelight to arrive at the depletion region with a sufficiently highelectric field in the semiconductor substrate. Upon arrival, thetransmitted light induces electron-hole pairs due to excitation ofphoto-energy and thereby produces current in the intrinsic depletionregion when light with varied wavelength transmits into the depletionregion.

[0009] In general, the depletion region of a CMOS image sensor is formedfar away from the surface of the semiconductor substrate. Since thewavelength of blue light, about 460 nanometers, is shorter than that ofred light and green light, most of the blue light passing through theCMOS image sensor cannot arrive at the depletion region. The poortransmittance of the blue light causes the semiconductor substrate toreceive insufficient light energy for current induction, leading toerroneous information.

[0010] Furthermore, a sensor region of a conventional CMOS image sensoris formed by implantation. The sensor region and the source/drain regionof the CMOS image sensor are formed at the same implanting step so thatthe sensor region and the source/drain region have the same impurityvarieties and the same implanting concentration. Arsenic (As) is usuallydoped into the substrate to form the source/drain region with aconcentration of about 1×10¹⁵ atoms/cm². As is heavier than phosphorous(P) and is doped into the substrate with a high energy of about 80 Kevso that the sensor region may be damaged from the high energy and theheavy atoms. The damage to the sensor region induces substrate leakage.

SUMMARY OF THE INVENTION

[0011] The invention provides a CMOS sensor. A gate oxide layer and agate electrode is formed and patterned on a provided substrate. Shallowfirst doped regions are formed in the substrate beside the gateelectrode. One of the shallow first doped regions is defined as asource/drain region. Another of the shallow first doped regions isdefined as a sensor region. A spacer is formed on the sidewall of thegate electrode. A first mask is provided to expose a part of thepredetermined sensor area. A second doped region is formed within thepredetermined sensor area by implanting. In the predetermined sensorarea, the second doped region is deeper than the first doped region. Thesensor region is composed of the first doped region and the second dopedregion. The shallow first doped region can enhance the response abilityfor blue light passing through the sensor region. The deep second dopedregion can maintain the response ability for red light and for greenlight passing through the sensor region.

[0012] A second mask is provided to expose the predeterminedsource/drain area. A second doped region within the predeterminedsource/drain area is thus formed by implanting. The first doped regionand the second doped region within the predetermined source/drain areaconstitute a source drain region with a lightly doped drain (LDD)region.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Other objects, features, and advantages of the invention willbecome apparent from the following detailed description of the preferredbut non-limiting embodiments. The description is made with reference tothe accompanying drawings in which:

[0014]FIG. 1 is a schematic, cross-sectional view of a portion of asemiconductor device showing a conventional CMOS sensor;

[0015]FIGS. 2A to 2E are schematic, cross-sectional views of a portionof a semiconductor device showing a CMOS sensor according to onepreferred embodiment of the invention; and

[0016]FIG. 3 is a top view showing the layout structure of a sensorregion of the CMOS sensor shown in FIG. 2E.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017]FIGS. 2A to 2E are schematic, cross-sectional views of a portionof a semiconductor device showing a CMOS sensor according to onepreferred embodiment of the invention.

[0018] In FIG. 2A, a substrate 200 having an isolating structure 202 isprovided. The substrate 200 is at least divided into two portions, 201 aand 201 b. Area 201 a is designated as a region in which a sensor regionwill be formed. Area 201 b is designated as a region in which asource/drain region will be formed. Furthermore, a gate oxide layer 204and a gate electrode layer 206 are formed and patterned on the substrate200. Although the gate electrode 206 shown in the figure is a mono-layerstructure, the gate electrode 206 actually may include a polysiliconlayer and a tungsten silicide layer.

[0019] An implanting process I₁ is performed to form first doped regions208 a and 208 b in the substrate 200 using the gate electrode 206 as amask with an energy of about 40 Kev. The first doped region 208 a ispositioned within the predetermined sensor area 201 a. The first dopedregion 208 b is positioned within the predetermined source/drain area201 b. The impurity implanted into the first doped regions 208 a, 208 bcomprises phosphorus (P) with a dosage of about 1×10¹³ atoms/cm².

[0020] In FIG. 2B, a spacer 210 is formed on the sidewall of the gateelectrode 206. A first mask 212 is provided over the substrate 200. Anopening 214 of the first mask exposes a part of the first doped region208 a within the predetermined sensor area 201 a.

[0021] In FIG. 2C, a implanting process 12 is performed using animplanting energy of about 70 Kev to implant impurities such asphosphorus through the opening 214 into the substrate 200 to form asecond doped region 216. Within the predetermined sensor area, thesecond doped region 216 is deeper than the first doped region 208 a. Animpurity concentration of the second doped region is about 1×10¹³atoms/cm². The first doped region 208 a and the second doped regionconstitute a sensor region with a dentoid profile. The first dopedregion 208 a is shallow for enhancing frequency response of blue light.The second doped region 216, which is deeper than the first doped region208 a, is used for enhancing frequency response of red light and greenlight.

[0022] In FIG. 2D, the mask 212 is removed. Another mask 218 with anopening 220 is provided over the substrate 200. The opening 220 exposesthe predetermined source/drain area 201 b of the substrate 200. Animplanting process 13 is performed with a implanting energy of about 80Kev to form a second doped region 222 in the predetermined source/drainarea 201 b using arsenic as an impurity. The concentration of theimpurity is about 1×10¹⁵ atoms/cm². Within the predeterminedsource/drain area 201 b of the substrate 200, the first doped region 208b and the second doped region 222 constitute a source/drain region witha lightly doped drain (LDD) structure.

[0023] In FIG. 2E, the mask 218 is removed. The sensor region and thesource/drain region of the CMOS sensor are complete. FIG. 3 is a topview showing the layout structure of a sensor region of the CMOS sensorshown in FIG. 2E. As shown in FIG. 3, the first doped region 208 a andthe second doped region 216 are alternatingly placed.

[0024] The invention provides a CMOS sensor structure comprising asource/drain region and a sensor region. The source/drain region isimplanted using arsenic for good conductivity. The sensor region isimplanted without arsenic for preventing the sensor region from beingdamaged by the heavy impurity, arsenic. Furthermore, the sensor regionhas a dentoid profile comprising a shallow first doped region and a deepsecond doped region.

[0025] While the invention has been described by way of example and interms of a preferred embodiment, it is to be understood that theinvention is not limited thereto. To the contrary, it is intended tocover various modifications and similar arrangements and procedures, andthe scope of the appended claims therefore should be accorded thebroadest interpretation so as to encompass all such modifications andsimilar arrangements and procedures.

What is claimed is:
 1. A CMOS sensor, comprising: a substrate; a gateelectrode formed on the substrate; a source/drain region formed in thesubstrate beside one side of the gate electrode, wherein thesource/drain further comprises a lightly doped drain region; and asensor region formed in the substrate beside another side of the gateelectrode, wherein the sensor region has a dentoid profile.
 2. The CMOSsensor according to claim 1 , wherein the source/drain region comprisesarsenic as an implanted impurity and the sensor region comprisesphosphorus as an implanted impurity.
 3. The CMOS sensor according toclaim 1 , wherein a concentration of arsenic in the source/drain regionis about 1×10¹⁵ atoms/cm².
 4. The CMOS sensor according to claim 1 ,wherein the lightly doped drain region of the source/drain regioncomprises phosphorus as an implanted impurity.
 5. The CMOS sensoraccording to claim 4 , wherein a concentration of phosphorus in thelightly doped drain region is about 1×10¹³ atoms/cm².
 6. The CMOS sensoraccording to claim 1 , wherein a concentration of phosphorus in thesensor region is about 1×10¹³ atoms/cm².
 7. The CMOS sensor according toclaim 1 , wherein the sensor region is at least composed of a firstdoped region and a second doped region, wherein the second doped regionis deeper than the first doped region.
 8. A sensor region of a CMOSsensor, comprising: a first doped region; and a second doped region;wherein the first doped region is formed by an implantation with aenergy of about 40 KeV and the second doped region is formed by animplantation with an energy of about 70 Kev so that the second dopedregion is deeper than the first doped region.
 9. The sensor regionaccording to claim 8 , wherein the first doped region and the seconddoped region comprise phosphorus with a concentration about 1×10¹³atoms/cm².